Rhythmic behavior production is essential for activities such as locomotion and breathing. A disruption to these systems can cause difficulties in executing even simple tasks. For this reason, extensive research has focused on the mechanisms behind rhythmic behaviors. Areas such as behavior-initiation and oscillation production are well understood in several species. However, there is a paucity of information regarding control of rhythmic behavior duration. How are nervous systems able to maintain the high level of excitation necessary to generate a sustained behavior? This proposal examines the maintenance of rhythmic swimming in the medicinal leech by testing the novel Swim-Maintenance Model. This model, based on preliminary and published data, proposes that multiple levels of control work in concert to sustain swimming. A novel cell, "OM", has been identified in a caudal midbody ganglion whose sustained excitation prolongs swimming indefinitely. In the caudal brain, several cells have been identified whose inhibition appears to be necessary for prolonged swimming, and whose excitation contributes to swim-termination. Sharp-electrode electrophysiology will be employed to classify the functional, cellular and circuit properties of these novel swim-control neurons. Our detailed knowledge of the cell-to-cell connections in leech swim-networks makes it an excellent system for computer modeling. Experimentally obtained data will be entered into the Neurodynamix II program to test if the Swim- Maintenance Model can, indeed, account for swim-maintenance. Completion of the proposed research will increase our understanding of how nervous systems sustain rhythmic behavior, forming a foundation for research on behavior maintenance in more complex species. Additionally, this proposal will fill an important gap in an otherwise well-characterized leech swim-system that can be used to study other areas such as spike coding, learning and memory, behavioral choice and in designing neuro-prosthetic devices.